Process for making an expansion screw for orthodontics and screw thus made

ABSTRACT

The present invention refers to a process for making an expansion screw and a screw thus made; the expansion screw ( 100, 100′ ) has two bodies ( 1, 2 ) that can be spaced apart by acting on the driving portion ( 6 ) of a double screw ( 3 ) housed within threaded central bores ( 30 ) provided in the two bodies ( 1, 2 ); each of the bodies ( 1, 2 ) has at least one bore ( 5, 50 ) parallel to the threaded bore; also provided is at least a hollow cylindrical guide ( 9 ) housed within a corresponding pair of parallel bores ( 5, 50 ) disposed on the two bodies ( 1, 2 ) in facing relationship; inserted in each guide ( 9 ) is a pin ( 4 ) for sliding therein; each guide ( 9 ) is fixed to one of the bodies, while the relevant pin ( 4 ) held therein is fixed to the other body, and the sliding of the pin ( 4 ) relative to the guide ( 9 ) takes place with friction owing to the force exerted upon assembly.

[0001] The present invention refers to a process for making an expansion screw for orthodontics and the orthodontic screw thus made.

[0002] Shown in FIGS. 1 and 2 of the attached drawings is an example of expansion screw of known type for orthodontic use. The screw comprises, substantially, a bearing structure with two main bodies (1, 2) provided with arms (8) which allow it to be associated to orthodontic bands (or other component of different conformation and/or function) to complete an orthodontic apparatus to be used for expanding the palate by suitably displacing the two bodies (1, 2) apart. The said bodies (1, 2) exhibit a central bore (30) with inner left-hand threading by one side and a right-hand threading by the opposite side. The two bodies are disposed in facing relationship and connected to each other by a double screw (3) having a central portion (6), with a plurality of driving bores (7), which is intended to result in an intermediate position between the same bodies (1, 2). The adjustment of the operating condition of the apparatus, during the orthodontic treatment, is made by means of a suitable tool inserted within the bores (7) of said central portion. The said screw (3) is double, that is, it exhibits two threaded spindles, extending from opposite sides with respect to the central portion (6), each spindle is intended for engagement with the central threaded bore of one of said bodies (1, 2), so as to causes them to move away from, or close to each other depending on the [direction of] rotation into which the central portion (6) is driven during said adjustment. Moreover, the said main bodies (1, 2) exhibit, in addition to the threaded central bore, two more bores (5) parallel to the central one and extending from opposite sides with respect to the axis thereof. The additional bores (5), without threading, have two cylindrical guide pins (4) going therethrough.

[0003] When using the expansion screws of a so-called “free” type, because they are not immersed in the resin of a palate plate, a clearance is provided between the pins (4) and the bores (5), as necessary for the sliding of same pins and as a consequence of the work tolerances. In order to limit the negative effects of such clearance, the coupling between the central screw (3) and the threaded bores (30) which accomodate it is made by friction. In practice, chemicals of so-called “thread-braking” type are used to create a friction between the threaded spindles of the central screw and the threads mating therein. In this way, unwanted rotating movements of the central screw, likely to endanger the success of the orthodontic treatment, are counteracted.

[0004] A drawback related to this production technique stems from the fact that a thus treated screw cannot be subjected afterwards to any thermal or chemical treatments (such as those for cleaning or welding the bands) which would imply the suppression of the involved products with consequent suppression of the braking function they perform. Besides, the thread-braking chemicals may result etched by substances present in the oral cavity and be dissolved.

[0005] A further drawback of the expansion screws of known type derives from the maximum extent of the expansion which cannot pass the phisical limits of the length of the guide pins. Moreover, when positioning the apparatus in its maximum expansion configuration, the same apparatus may result somewhat unstable, since the pins (4) are almost entirely withdrawn from the relevant bores (5) (see FIG. 2).

[0006] A further (known) technique provides for deforming the threading of the nut screw which accomodates the threaded spindles of the central screw. However, in practice, it is impossible to ensure the same deformation all the time, so that, in some cases, a seizure effect may occur in the system after a brief period of use or, in other cases, the friction may result insufficient in relation to the work tolerances of the mating elements.

[0007] The main object of the present invention is to overcome the said drawbacks.

[0008] This result has been achieved, according to the invention, by providing an operating process and an orthodontic expansion screw having the characteristics indicated in the independent claims. Further characteristics being set forth in the dependent claims.

[0009] The advantages deriving from the present invention lie essentially in the fact that it is possible to ensure optimal stability and rigidity of the expansion screw when this is in use; that there is obtained a relatively wide expansion in relation to the overall dimensions of the screw; that the screw results safely retained in spite of its limited dimensions and expansion extent obtained (that is, with equal expansion, there is obtained a minor bulkiness of the guides and a larger mating surface, with respect to the known solutions); that the present screw can be subjected to phisical and chemical treatments of sterilization or others, while maintaining its characteristics even after a prolonged service life.

[0010] These and other advantages and characteristics of the present invention will be best understood by anyone skilled in the art from a reading of the following description in conjunction with the attached drawings given as a practical exemplification of the invention, but not to be considered in a limitative sense, wherein:

[0011]FIGS. 1 and 2 are schematic sectional views with parts taken away of one embodiment of the orthodontic expansion screw of known type, respectively in a minimum and maximum bulkiness configuration;

[0012]FIG. 3 is a plan view, with parts taken away and in section, of one example of application of the present invention to a dental arch;

[0013]FIGS. 4 and 5 are schematic plan views in horizontal section, with parts taken away, of one embodiment of the orthodontic expansion screw according to the present invention, respectively in a minimum and maximum bulkiness configuration;

[0014]FIGS. 6A, 6B, 6C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step of a possible embodiment of the process according to the invention;

[0015]FIGS. 7A, 7B, 7C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step subsequent to that of FIGS. 6A-6C;

[0016]FIGS. 8A, 8B, 8C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step subsequent to that of FIGS. 7A-7C;

[0017]FIG. 8D shows a plan view in horizontal section of the embodiment of FIG. 8A;

[0018]FIG. 8E is an enlarged detail of the area encircled by a broken line in FIG. 8D;

[0019]FIGS. 9A, 9B, 9C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step subsequent to that of FIGS. 8A8C;

[0020]FIGS. 10A, 10B, 10C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step of a further possible embodiment of the process according to the invention;

[0021]FIGS. 11A, 11B, 11C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step subsequent to that of FIGS. 10A-10C;

[0022]FIGS. 12A, 12B, 12C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step subsequent to that of FIGS. 11A-11C;

[0023]FIGS. 13A, 13B, 13C show, respectively, a plan view with parts taken away, a right side view and a left side view relating to a step subsequent to that of FIGS. 12A-12C;

[0024]FIGS. 14 and 15 are schematic plan views in horizontal section, with parts taken away, of a further embodiment of the orthodontic expansion screw according to the present invention, respectively in a minimum and maximum bulkiness configuration;

[0025]FIG. 16 is an enlarged detail of the area encircled by a broken line in FIG. 15;

[0026]FIG. 17 is a plan view in section, with parts taken away, of one example of application of the screw of FIGS. 14-16.

[0027] With reference to the figures of the attached drawings, which figures, for sake of clarity, are not all in the same scale, the process in question can be implemented for making an orthodontic expansion screw of a type comprising two main bodies (1, 2) having a threaded central bore (30) and two side bores (5, 50). The threaded bores (30) are able to receive a double screw (3), while the side bores (5) are provided for housing two cylindrical guide pins (4).

[0028] The attached drawings show two possible embodiments of the present method, respectively in FIGS. 3-9C and 10A-13C. In the description that follows, although reference is made directly to FIGS. 3-13C, it is understood that a similar process makes it possible to provide also the other embodiment of screw (100′) represented in FIGS. 14-17 and having only two bores (5, 50).

[0029] With reference to the figures showing the nonlimitative examples of the accompanying drawings, the process includes the operating steps described herebelow.

[0030] The two main bodies (1, 2) are made by forming on each of them the threaded central bore (30) and two side bores parallel to the central one, so that the whole of the two bodies exhibits two threaded central bores (30), intended to accomodate the central screw (3), and four side bores (5, 50). In the examples, each of the two bodies (1, 2) has two arms (8) to be associated either with orthodontic bands (80) or with other suitable means for fixing and/or connecting the teeth of a dental arch. In the case of the screw (100′) depicted in FIGS. 14-17, only one, non threaded bore will be formed on the bodies (1, 2); in particular, a one-diameter bore (5) on the body (1) and a diameter-varying bore (50) on the body (2). Moreover, only one arm (8) is provided for each of bodies (1, 2).

[0031] In particular, the central bore (30) is formed in the same way as provided by the known technique, inasmuch as it is the double screw (3) to be associated therewith. In the example of FIGS. 3-9C, two differently shaped side bores (5, 50) are formed for each of the bodies (1, 2): one of said bores (5), for example the one located in the upper part of the body (1), like the one located in the lower part of body (2), has a constant diameter (D5); the other bore (50) of each body has a constant diameter (D5), the same as diameter (D5) of the first bore (5), in correspondence of a first section (54) and for an extension of predetermined length, while exhibiting a minor diameter (D55) in correspondence of the opposite section (55), that is, of the section which results on the outer wall (10, 20) of body (1, 2). In the example of FIGS. 10A-13C, instead, the body (1) has both bores (50) with varying diameter, while the body (2) has the two bodies of constant diameter (5). In the example of FIGS. 14-17, in which the screw (100′) is represented in assembled condition, the body (1) has a bore (5) of constant diameter, while the body (2) has a bore (50) of varying diameter.

[0032] The two bodies (1, 2) are disposed in facing relationship, so as to allign a constant-diameter side bore (5) of one body with a diameter-varying bore (50) of the other body, by screwing the double screw (3) as necessary within the corresponding threaded bores (30) formed in the two bodies (1, 2) and thus connecting the latter to each other.

[0033] Then two cylindrical hollow guides (9) are used which have, along their longitudinal development, a constant inner diameter substantially corresponding to the said minor diameter (D55), while, in correspondence of a first or leading end (91) they exhibit, at lest internally, a reduced diameter. Each cylindrical hollow guide (9) is inserted, on the side of its leading end (91), into the constant-diameter bore (5) of one body (1, 2) and, then, into the bore (50) of the other body. Obviously, the outer diameter of the guides (9) is such as to allow the insertion thereof within the bores (5, 50). In practice, each guide (9) goes through the body having constant-diameter bore (5) to enter the opposite bore (50) formed in the other body. This step is illustrated in FIGS. 6A-6C for the first example, and in FIGS. 10A-10C for the second wherein the arrow (F) indicates the directions of insertion of the guides (9).

[0034] Thereafter, each cylindrical guide (9) is fixed to one of said bodies (1, 2) in correspondence of the opposite or trailing side (90). The fixing can be carried out by laser, as best visible in FIGS. 7B, 7C and 11C, wherein the laser-welded spots are indicated with numeral (59). The fixing of the hollow guides (9) to the bodies (1, 2) is made in correspondence of the trailing end (90) thereof, this end being disposed substantially flush with the outer wall (10, 20) of the body to which it is fixed.

[0035] Afterwards, a cylindrical pin (4) is inserted into each cylindrical hollow guide (9) through the relevant trailing end (90), as indicated by the arrows (G) in FIGS. 8A and 12A. The outer diameter (D4) of each pin (4) corresponds substantially to said minor diameter (D55) of diameter-varying bores (50) formed in said bodies (1, 2); each pin (4) is inserted into the relevant guide (9) as far as its leading end (41) reaches the end (91) of same guide. The said pins (4) are pushed further through said end (91) of the hollow guides (9), with some force to win the interference due to the difference of the respective diameters (D4, D91), and as far as to have the leading end (41) of pins (4) in correspondence of the outer section (55) of the relevant bore (50). This step being shown in FIGS. 8A and 12A.

[0036] The leading ends of the two pins (4) are then fixed to the respective bodies (1, 2) in correspondence of said leading section (55) by laser-welding, for example, as best shown in FIGS. 9B, 9C and 13B wherein numeral (49) indicates the laser-welding spots.

[0037] In this way there is obtained an expansion screw (100) which has two bodies (1, 2) that can be spaced apart by means of a known per se driving tool fitted into the bores (7) of the driving portion (6) of double screw (3) to drive the latter into rotation, according to an expansion program as established by the doctor; each of said bodies (1, 2) of the expansion screw (100) has two side bores which may be either different to each other, as in the example relevant to the first embodiment of the method (see FIGS. 3-9C), or equal to each other, as in the second example (see FIGS. 10A-13C). In each of said side bores (5, 50) a guide (9) is provided for a pin (4) to slide therein. Each guide (9) is fixed to one of said bodies, while the relevant pin (4) present therein is fixed to the other body; the sliding of the pin relative to the guide takes place with some friction owing to the force exerted upon assembly.

[0038] With reference to the example of FIGS. 14-17, the expansion screw (100′) thus obtained exhibits two bodies (1, 2) displaceable from each other by means of a drive tool, known per se, fitted into the bores (7) of the driving portion (6) of double screw (3) to drive the latter into rotation, according to an expansion program as established by the doctor; each of said bodies (1, 2) of the expansion screw (100) has one bore: in particular, the body (1) has a constant-diameter bore (5) and the body (2) has a diameter-varying bore (2). Within the bores (5, 50) there is a guide (9) for a pin (4) to slide therein. The guide (9) is fixed to the body (1), while the relevant pin (4) held therein is fixed to the other body (2); numerals (59) and (49) in FIG. 15 indicate the possible sites for fixing. The sliding of the pin relative to the guide takes place with some friction owing to the force exerted upon assembly.

[0039] It will be appreciated that the stability and rigidity of the thus made screws (100, 100′) are remarkable even in case the central screw (3) is not subjected to specific treatments. The friction between each guide (9) and relevant pin (4) is such as to prevent spontaneous and undesired rotations of the screw (3) when in operative condition. It should be noted, in particular, the fact that the association between guides and pins is made with tolerances hardly obtainable by other working processes, and this because each pin is in fact inserted into its relevant guide by force, as above mentioned.

[0040] Moreover, the expansion of screw (100) is quite broad, as the lengths of the guides and those of the pins sum up by giving rise to a sort of telescopic connection between the bodies (1, 2), which determines coupling surfaces much extended between the guides and the pins also under maximum expansion condition.

[0041] Under a condition of maximum expansion, as shown in FIGS. 5 and 15, it is not possible either to contract or expand any longer the screw, since the spindles of the central screw (3) result outside the threadings (30). As best viewable in the detail of FIG. 16, when the screw (100, 100′) is in its condition of maximum expansion, the two ends of screw (3) lie in an inner, non-threaded, terminal region (33) of the bore (30). However, even when taking such configuration, the screw results all the same stable and rigid thanks to the length of pins (4) which allows them to remain housed within the guides (9). Accordingly, the advantages offered by the present screw make it possible to ensure a correct and safe orthodontic treatment while preventing any accidental disassembly of the same screw within the patient's mouth.

[0042] The construction details may vary in any equivalent way as far as the shape, dimensions, elements disposition, nature of the used materials are concerned, without nevertheless departing from the scope of the adopted solution idea and, thereby, remaining within the limits of the protection granted to the present patent. 

1. Process for making an orthodontic expansion screw of a type comprising two main bodies (1, 2) having at least one threaded bore (30) and at least one bore parallel to the threaded one, a double screw (3) with a driving portion (6) the spindles of same screw fitting into the threaded bores (30) provided in said bodies (1, 2), and at least a guide pin (4) to be disposed coaxial to said parallel bore, said process comprising the steps of making said main bodies (1, 2) by forming on each of them the threaded bore (30) and at least a bore parallel thereto, and disposing the two bodies (1, 2) in facing relationship so as to align said opposite parallel bores by screwing, as necessary, the double screw (3) within the corresponding theraded bores (30) provided on the two bodies (1, 2) thereby connecting the latter to each other, characterized in that it comprises the following operative steps: inserting into each pair of thus facing side bores (5, 50) a hollow cylindrical guide (9) having a leading end (91) and a trailing end (90); fixing the trailing end (90) of each hollow cylindrical guide (9) to the outer wall (10, 20) of one of said bodies (1, 2); inserting into each hollow cylindrical guide (9), through the said trailing end (90), a cylindrical pin (4) of a diameter (D4) slightly greater than the diameter (D91) of the leading end (91) of guide (9), as far as to have the leading end (41) of said pin (4) in correspondence of the leading end (91) of guide (9); pushing each pin (4), through said leading end (91) with some force to win the interference due to the difference of the respective diameters (D4, D91), and as far as to have the leading end (41) of each pin (4) in correspondence of the outer section (55) of the bore (50) opposite to the trailing end (90) of the respective guide (9); fixing said leading end of pin (4) to one of said bodies (1, 2) in correspondence of said section (55) intended to receive the leading end (41) of pin (4).
 2. Process according to claim 1, especially for making a screw comprising two main bodies (1, 2) having a threaded central bore (30) and two side bores parallel to the central one, a double screw (3) with a driving portion (6), the spindles of same screw engaging the threaded bores (30) provided in said bodies (1, 2), and a pair of guide pins (4) to be disposed coaxial to said side bores, the process comprising the steps of making the said main bodies (1, 2) by forming on each of them the threaded central bore (30) and two side bores and disposing the two bodies (1, 2) in facing relationship so as to align the said side bores opposite to each other, by screwing as necessary the double screw (3) within the corresponding theraded bores (30) provided on the two bodies (1, 2) thereby connecting the latter to each other, characterized in that it comprises the following operative steps: inserting into the pairs of thus facing side bores (5, 50) two hollow cylindrical guides (9) having a leading end (91) and a trailing end (90); fixing the trailing end (90) of each hollow cylindrical guide (9) to the outer wall (10, 20) of one of said bodies (1, 2); inserting into each hollow cylindrical guide (9), through the said trailing end (90), a cylindrical pin (4) of a diameter (D4) slightly greater than the diameter (D91) of the leading end (91) of guide (9), as far as to have the leading end (41) of said pin (4) in correspondence of the leading end (91) of guide (9); pushing the said pins (4), through said leading end (91) with some force to win the interference due to the difference of the respective diameters (D4, D91) and as far as to have the leading end (41) of the pins (4) in correspondence of an outer section (55) of the bore (50) opposite to the trailing end (90) of the respective guide (9); fixing said leading end of pin (4) to one of said bodies (1, 2) in correspondence of said section (55) tended to receive the leading end (41) of pin (4).
 3. Process for making an orthodontic expansion screw comprising two main bodies (1, 2) having one threaded central bore (30) and at least one bore parallel to the central one, a double screw (3) with a driving portion (7) and whose spindles engage the threaded bores (30) provided in said bodies (1, 2), and at least one guide pins (4) to be disposed coaxial to said side bores, characterized in that it comprises the following operative steps: making said main bodies (1, 2) by forming thereon the threaded bore (30) and at least a side bore so as to achieve, in all, two threaded bores (30), at least a parallel bore (5) of constant diameter (D5), and at least a parallel bore (50) of varying diameter which exhibits a constant diameter (D5) in correspondence of a first section (54) and by an extent of predetermined length, and a minor diameter (D55) in correspondence of the opposite section (55) which is intended to result externally disposed; disposing the two bodies (1, 2) facing to each other so as to line up at least a bore (5) of constant diameter with one (50) of varying diameter, by screwing as necessary the double screw (3) into the corresponding threaded bores (30) provided on the two bodies (1, 2), thereby connecting the latter to each other; making at least a hollow cylindrical guide (9), having a constant inner diameter corresponding substantially to said minor diameter (D55) along its longitudinal development, and a reduced diameter, at least internally, in correspondence of a first or leading end (91); inserting each hollow cylindrical guide (9), by the side of its leading end (91), into a bore (5) of constant diameter provided on one body (1, 2) and, thereafter, into the corresponding bore (50) of varying diameter provided on the other body (1, 2); fixing each cylindrical guide (9) to one of said bodies (1, 2), in correspondence of the opposite or trailing end (90); inserting into each hollow cylindrical guide (9), through the said trailing end (90), a cylindrical pin (4) whose external diameter (D4) corresponds substantially to the minor diameter (D55) of diameter-varying bore (50), as far as the leading end (41) of said pin (4) reaches the said leading end (91) of guide (9); pushing each pin (4), through said leading end (91) with some force to win the interference due to the difference of the matching diameters (D4, D91), as far as to have the leading end (41) of each pin (4) in correspondence of the outer section (55) of the bore (50); fixing said leading end of pin (4) to one of said bodies (1, 2) in correspondence of said external section (55).
 4. Process according to claim 3, especially for making a screw comprising two main bodies (1, 2) having a threaded central bore (30) and two side bores parallel to the central one, a double screw (3) with a driving portion (7) and whose spindles engage the threaded bores (30) provided in said bodies (1, 2), and a pair of guide pins (4) to be disposed coaxial to said side bores (5), characterized in that it comprises the operating steps of: making the said main bodies (1, 2) by forming on each of them the threaded central bore (30) and two side bores, so as to achieve, in all, two threaded central bores (30), two side bores (5) of constant diameter (D5) and two side bores (50) of varying diameter which exhibits a constant diameter (D5) in correspondence of a first section (54) and by an extent of predetermined length, and a minor diameter (D55) in correspondence of the opposite section (55) which is intended to result externally disposed; disposing the two bodies (1, 2) facing to each other so as to line up a side bore (5) of constant diameter with one (50) of varying diameter, by screwing as necessary the double screw (3) into the corresponding threaded bores (30) provided on the two bodies (1, 2), thereby connecting the latter to each other; making two hollow cylindrical guides (9), having a constant inner diameter corresponding substantially to said minor diameter (D55) along its longitudinal development, and a reduced diameter, at least internally, in correspondence of a first or leading end (91); inserting each hollow cylindrical guide (9), by the side of its leading end (91), into a bore (5) of constant diameter provided on one body (1, 2) and, thereafter, into the corresponding bore (50) of varying diameter provided on the other body (1, 2); fixing each cylindrical guide (9) to one of said bodies (1, 2), in correspondence of the opposite or trailing end (90); inserting into each hollow cylindrical guide (9), through the said trailing end (90), a cylindrical pin (4) whose external diameter (D4) corresponds substantially to the minor diameter (D55) of diameter-varying bore (50), as far as the leading end (41) of said pin (4) reaches the said leading end (91) of guide pushing each pin (4), through said leading end (91) with some force to win the interference due to the difference of the matching diameters, as far as to have the leading end (41) of each pin (4) in correspondence of the outer section (55) of the bore (50); fixing said leading end of pin (4) to one of said bodies (1, 2) in correspondence of said external section (55).
 5. Process according to claim 4, characterized by forming a side bore (50) of varying diameter for each body (1, 2).
 6. Process according to claim 4, characterized in that the side bores (50) of varying diameter are both formed on the same body (1).
 7. Process according to one or more preceding claims, characterized by fixing the guides (9) to the bodies (1, 2) by laser-welding.
 8. Process according to one or more preceding claims, characterized by fixing the pins (4) to the bodies (1, 2) by laser-welding.
 9. Orthodontic expansion screw of a type comprising two main bodies (1, 2) having a threaded bore (30) and at least one bore parallel to the threaded one, a double screw (3) with a driving portion (6) and whose spindles fit into the threaded bores (30) provided in said bodies (1, 2), and at least a guide pin (4) to be disposed coaxial to said parallel bores (5), characterized in that it is made by the process according to one or more preceding claims.
 10. Orthodontic expansion screw according to claim 9, characterized in that it is of a type comprising two main bodies (1, 2) having one threaded central bore (30) and two side bores (5, 50) parallel to the central one, a double screw (3) with a driving portion (7) and whose spindles engage the threaded bores (30) provided in said bodies (1, 2), and a pair of guide pins (4) to be disposed coaxial to said side bores (5, 50).
 11. Orthodontic expansion screw comprising two main bodies (1, 2) having one threaded central bore (30) and at least one bore parallel to the central one, a double screw (3) with a driving portion (6) and whose spindles engage the threaded bores (30) provided in said bodies (1, 2), and at least one guide pin (4) to be disposed coaxial to said side bores, characterized in that it comprises at least one hollow cylindrical guide (9) steadily housed within said at least one parallel bore (5, 50), fixed to one of said bodies (1, 2) and freeling going through the other, and in that said guide pin (4) in inserted with interference into said guide (9) and is fixed to the body which is freely crossed by the guide (9) having same pin inserted thereinto.
 12. Orthodontic expansion screw according to claim 11, especially of a type comprising two main bodies (1, 2) having one threaded central bore (30) and two side bores parallel to the central one, a double screw (3) with a driving portion (6) and whose spindles engage the threaded bores (30) provided in said bodies (1, 2), and a pair of guide pins (4) to be disposed coaxial to said side bores, characterized in that it comprises a pair of hollow cylindrical guides (9) steadily housed within said side bores (5, 50), being fixed to one of said bodies (1, 2) and freeling going through the other, and in that said guide pins (4) are inserted with interference into said guides (9) and are fixed to the body which is freely crossed by the guides (9) having same pins inserted thereinto.
 13. Expansion screw according to one or more claims 9 to 12, characterized in that on said bodies (1, 2) there are provided, in all, at least a parallel bore (5) with constant diameter (D5) and a bore (50) with varying diameter which has a constant diameter (D5) in correspondence of a first section (54) and by an extent of predetermined length, and a minor diameter (D55) in correspondence of the opposite section (55) which is intended to result externally disposed.
 14. Expansion screw according to claims 10 and/or 12, characterized in that on said bodies (1, 2) there are provided, in all, two side bores (5) with constant diameter (D5) and two side bores (50) with varying diameter which have a constant diameter (D5) in correspondence of a first section (54) and by an extent of predetermined length, and a minor diameter (D55) in correspondence of the opposite section (55) which is intended to result externally disposed.
 15. Expansion screw according to claim 14, characterized in that the two bores with varying diameter (50) are provided one for each body (1, 2).
 16. Expansion screw according to claim 14, characterized in that the two bores with varying diameter (50) are provided on the same body (1, 2).
 17. Expansion screw according to claim 14, characterized in that the said hollow cylindrical guides (9) have constant diameter, substantially corresponding to said minor diameter (D55) along their longitudinal development and a reduced diameter (D91), ate least internally, in correspondence of a first or leading end (91).
 18. Expansion screw according to claim 14, characterized in that the said pins (4) have an outer diameter (D4) substantially corresponding to said minor diameter (D55) of the diameter-varying bore. 